This invention relates to the production of thin steel strip in a strip caster, particularly a twin roll caster.
In a twin roll caster, molten metal is introduced between a pair of contra-rotated horizontal casting rolls which are cooled so that metal shells solidify on the moving roll surfaces and are brought together at the nip between them to produce a solidified strip product delivered downwardly from the nip between the rolls. The term xe2x80x9cnipxe2x80x9d is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel from which it flows through a metal delivery nozzle located above the nip so as to direct it into the nip between the rolls, so forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip and extending along the length of the nip. This casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the rolls so as to dam the two ends of the casting pool against outflow, although alternative means such as electromagnetic barriers have also been proposed.
When casting steel strip in a twin roll caster the strip leaves the nip at very high temperatures of the order of 1400xc2x0 C. and if exposed to air, it suffers very rapid scaling due to oxidation at such high temperatures.
It has therefore been proposed to shroud the newly cast strip within an enclosure containing a non-oxidising atmosphere until its temperature has been reduced significantly, typically to a temperature of the order of 1200xc2x0 C. or less so as to reduce scaling. One such proposal is described in U.S. Pat. No. 5,762,126 according to which the cast strip is passed through a sealed enclosure from which oxygen is extracted by initial oxidation of the strip passing through it thereafter the oxygen content in the sealed enclosure is maintained at less than the surrounding atmosphere by continuing oxidation of the strip passing through it so as to control the thickness of the scale on the strip emerging from the enclosure. The emerging strip is reduced in thickness in an inline rolling mill and then generally subjected to forced cooling, for example by water sprays and the cooled strip is then coiled in a conventional coiler.
Previously, it has been proposed in strip casting to cool the strip through the austenite transformation zone by subjecting the strip to water sprays. Such water sprays are capable of producing maximum cooling rates of the order of 90xc2x0 C./sec. The cooling intensity has a dramatic effect on the final strip microstructure. It is possible to achieve a remarkable degree of hardenability in typical low carbon steel chemistry by employing accelerated cooling rates, to promote the formation of low temperature transformation products which enables an increased range of strip products to be produced, particularly with a range of yield strength and hardness, even in the case where inline hot reduction has refined the xe2x80x98as castxe2x80x99 microstructure.
According to the disclosure there is provided a method of producing steel strip comprising:
continuously casting molten plain carbon steel into a strip of not more than 5 mm in thickness and including austenite grains;
passing the strip through a roll mill in which the strip is hot rolled to produce a reduction in strip thickness by more than 15%;
cooling the strip to transform the strip austenite to ferrite within the temperature range of 850xc2x0 C. to 400xc2x0 C. at a cooling rate of not less than 90xc2x0 C./sec.
The strip is continuously cast by supporting a casting pool of molten steel on a pair of chilled casting rolls forming a nip between them and the solidified strip is produced by rotating the rolls in mutually opposite directions such that the solidified strip moves downwardly from the nip.
The cooling rate is illustratively in the range of 100xc2x0 C./sec to 300xc2x0 C./sec. The strip may be cooled through the transformation temperature range within between 850xc2x0 C. and 400xc2x0 C. and not necessarily through that entire temperature range at such a cooling rate. The precise transformation temperature range will vary with the chemistry of the steel composition and processing characteristics.
The term xe2x80x9clow carbon steelxe2x80x9d is understood to mean steel of the following composition, in weight percent:
C: 0.02-0.08
Si: 0.5 or less;
Mn: 1.0 or less;
residual/incidental impurities: 1.0 or less; and
Fe: balance
The term xe2x80x9cresidual/incidental impuritiesxe2x80x9d covers levels of elements, such as copper, tin, zinc, nickel, chromium, and molybdenum, that may be present in relatively small amounts, not as a consequence of specific additions of these elements but as a consequence of standard steel making. Elements may be present as a result of using scrap steel to produce plain carbon steel.
The low carbon steel may be silicon/manganese killed and may have the following composition by weight:
Silicon/manganese killed steels are particularly suited to twin roll strip casting. A silicon/manganese killed steel will generally have a manganese content of not less than 0.20% (typically about 0.6%) by weight and a silicon content of not less than 0.10% (typically about 0.3%) by weight.
The low carbon steel may be aluminum killed and may have the following composition by weight:
The aluminum killed steel may be calcium treated.
The method presently disclosed enables the production of steel strip with yield strength significantly greater than 450 MPa. More specifically, strip may be produced with a yield strength in the range of 450 to in excess of 700 MPa by cooling rates in the range of 100xc2x0 C./sec to 300xc2x0 C./sec. However, the aluminum killed steels will be generally 20 to 50 MPa softer than the silicon/manganese killed steels.
In one embodiment, a method comprises guiding the strip passing from the casting pool through an enclosure containing an atmosphere which inhibits oxidation of the strip surface and consequent scale formation.
The atmosphere in said enclosure may be formed of inert or reducing gases or it may be an atmosphere containing oxygen at a level lower than the atmosphere surrounding the enclosure.
The atmosphere in the enclosure may be formed by sealing the enclosure to restrict ingress of oxygen containing atmosphere, causing oxidation of the strip within the enclosure during an initial phase of casting thereby to extract oxygen from the sealed enclosure and to cause the enclosure to have an oxygen content less than the atmosphere surrounding the enclosure, and thereafter maintaining the oxygen content in the sealed enclosure at less than that of the surrounding atmosphere by continuous oxidation of the strip passing through the sealed enclosure thereby to control the thickness of the resulting scale on the strip.
The strip may be passed through a rolling mill in which it is hot rolled with a reduction in thickness of up to 50%.
In one embodiment, after hot rolling, the strip passes on to a run-out table with cooling means operable to cool the cast strip transforming the strip from austenite to ferrite in a temperature range of 850xc2x0 C. to 400xc2x0 C. at a cooling rate of not less than 90xc2x0 C./sec.